Microparticle Brownian Motion near an Air-Water Interface Governed by Direction-Dependent Boundary Conditions

TL;DR

This study measures the 3D Brownian motion of particles near an air-water interface, revealing boundary condition effects that challenge existing models and depend on surface-active particles.

Contribution

It provides the first detailed experimental analysis of particle dynamics at nanometer distances from an air-water interface, highlighting boundary condition effects and introducing a new model.

Findings

Parallel motion aligns with slip boundary predictions

Normal motion aligns with no-slip boundary predictions

Surface-active particles influence boundary conditions

Abstract

Although the dynamics of colloids in the vicinity of a solid interface has been widely characterized in the past, experimental studies of Brownian diffusion close to an air-water interface are rare and limited to particle-interface gap distances larger than the particle size. At the still unexplored lower distances, the dynamics is expected to be extremely sensitive to boundary conditions at the air-water interface. There, ad hoc experiments would provide a quantitative validation of predictions. Using a specially designed dual wave interferometric setup, the 3D dynamics of 9 micrometers diameter particles at a few hundreds of nanometers from an air-water interface is here measured in thermal equilibrium. Intriguingly, while the measured dynamics parallel to the interface approaches expected predictions for slip boundary conditions, the Brownian motion normal to the interface is very close to the predictions for no-slip boundary conditions. These puzzling results are rationalized considering current models of incompressible interfacial flow and deepened developing an ad hoc model which considers the contribution of tiny concentrations of surface active particles at the interface. We argue that such condition governs the particle dynamics in a large spectrum of systems ranging from biofilm formation to flotation process.